1. Field of the Invention
The present invention generally relates to the manufacture of very large scale integrated (VLSI) circuit devices and, more particularly, to the preparation of X-ray masks used in lithographic processes to transfer patterns onto a devices substrate in the manufacture of semiconductor memory and logic chips.
2. Background Description
Manufacturing of semiconductor devices is dependent upon the accurate replication of computer aided design (CAD) generated patterns onto the surface of a device substrate. The replication process is typically performed using a lithographic process followed by a variety of subtractive (etch), additive (deposition) and material modification (e.g., oxidations, ion implants, etc.) processes.
X-ray mask images are made the same size as the final images on the wafer, whereas most optical masks have images four or five times the size of the images on the wafer. The optical stepper uses a precision reduction lens to reduce the mask image to the image that is exposed on the wafer. Because of the one-to-one relationship of the image on an X-ray mask and the image formed on the wafer, any position errors in building the X-ray mask are replicated one for one onto the wafer. Thus, the position accuracy requirements for the fabrication of X-ray masks are very difficult to achieve.
X-ray masks are fabricated using an electron beam (E-beam) lithography system. There are several different E-beam lithography system architectures. One architecture that is used by several manufacturers, including IBM Microelectronics, is a shaped beam with a deflection field made up of subfields. Position errors in this type of architecture occur at spot boundaries, subfield boundaries, and field boundaries. One method to reduce the random errors at these boundaries is multipass writing. Each pattern of the multipass write is shifted relative to the field and subfield boundaries. The exposure of each pass dose is reduced such that the sum of the multiple exposure doses is equivalent to a fully exposed single dose. The E-beam resist is partially exposed on each exposure pass which has the effect of fully exposing the resist at the average position. This averaging action reduces the effect of the random position noise. The shifting of the pattern relative to the various boundaries reduces jogs at those boundaries.
Before a pattern can be written on the E-beam system, the design must be converted into E-beam drive code. One implementation of multipass writing uses a parameter in the program that converts the design data to E-beam drive code to shift the pattern with respect to the field and subfield boundaries. The mask stage addresses are adjusted so that each writing pass will overlay on the mask being written. A second implementation changes the size of the fields and the size of the subfields on the E-beam exposure tool. The program that converts the design data into E-beam drive code is run with parameters to match the tool setups. Both of these methods require running the design data through software programs that convert the design data to electron beam drive code multiple times, once for each writing pass. The computer time cost of running the conversion program is high. A typical large memory art may cost $5,000 to process. Another cost is the cost of the hardware to store the E-beam drive code. To store the drive code for each pass may require as much as a quarter of a gigabyte of disk space. To write four passes (typical in current multipass implementations) would require four different copies of the E-beam drive code.